Last time, we talked about what a rel=canonical request is and how to implement one on pages that may contain duplicate content (as happens frequently with retail sites, for instance). Now for the “when.” When is it appropriate to use a rel=canonical as opposed to a 301 redirect? Does it matter one way or the other, as far as SEO goes?
A 301 redirect is a way that webmasters use to indicate that content has moved to a new URL. It’s a forwarding address, in other words, and most users don’t realize that they have been redirected at all. They are useful when you are moving sites permanently or when you have multiple homepages that you want to be rerouted to the correct URL. For instance, if your site is ABCD.com, it could also be listed as ABCD.com/home.html or ABCD.com/index.html. Your 301 could redirect users to the simple ABCD.com.
In the latter case, you would want to use rel=canonical if each version of the homepage served a different purpose. For instance, one could be targeted for users in City X, while another was geared for users in City Z.
Now, if you are concerned about your link value, a 301 may be better. The loss of value when you go to a rel=canonical or 301 is the same, but a 301 means you only have to take that loss once. After you put the redirect in place, users are automatically routed to the version you want, and if they link to your page, that link “juice” goes to your canonical, or preferred version. On the other hand, if you use rel=canonical, both URLs are still resolving and you are losing that link value.
Another rel=canonical no-no is using the tag across your entire site and requesting that Google index just one page. For instance, if you did this and asked Google to only consider your homepage. That will decrease rankings in a hurry as the search engine can de-index those portions of your site.
Both 301s and rel=canonical can be invaluable tools, but they can also cause inexperienced sites to lose ranking and link value. Before you go with one or the other, make sure it will accomplish what you want and will not lower your rank.